Method and arrangement for tracking movement relative to a surface

ABSTRACT

In an embodiment, a method of tracking movement relative to a surface involves comparing at least some data elements of a first array with at least some of data elements of a second array if it is determined, from an examination of the first array of data elements, that a characteristic condition does not exist in the first array of data elements, wherein the characteristic condition is related to the uniformity of at least a majority of the data elements of the first array.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of Ser. No. 11/154,008, filed Jun.15, 2005, which is a continuation of Ser. No. 10/217,725, filed Aug. 12,2002, now U.S. Pat. No. 6,950,094, which is a continuation of Ser. No.09/753,805, filed Jan. 2, 2001, now U.S. Pat. No. 6,433,780, which is acontinuation of Ser. No. 09/052,046, filed Mar. 30, 1998, now U.S. Pat.No. 6,281,882 which is a continuation-in-part of Ser. No. 08/540,355,filed Oct. 6, 1995, now U.S. Pat. No. 5,786,804.

REFERENCE TO RELATED PATENTS

This application is related to the subject matter described in thefollowing two U.S. Pat. Nos. 5,578,813 filed 2 Mar. 1995, issued 26 Nov.1996 and entitled FREEHAND IMAGE SCANNING DEVICE WHICH COMPENSATES FORNON-LINEAR MOVEMENT; and 5,644,139, filed 14 Aug. 1996, issued 1 Jul.1997 and entitled NAVIGATION FOR DETECTING MOVEMENT OF NAVIGATIONSENSORS RELATIVE TO AN OBJECT. Both of these patents have the sameinventors: Ross R. Allen, David Beard, Mark T. Smith and Barclay J.Tullis, and both Parents are assigned to Hewlett-Packard Co. Thisapplication is also related to the subject matter described in U.S. Pat.No. 5,786,804 filed 6 Oct. 1995, entitled METHOD AND SYSTEM FOR TRACKINGATTITUDE, issued 28 Jul. 1998, and also assigned to Hewlett-Packard Co.These three patents describe techniques of backing position movement.Those techniques are a component in the preferred embodiment describedbelow. Accordingly, U.S. Pat. Nos. 5,578,813, 5,644,139 and 5,786,804are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The use of a hand operated pointing device for use with a computer andits display has become almost universal. By far the most popular of thevarious devices is the conventional (mechanical) mouse. A conventionalmouse typically has a bottom surface carrying three or more downwardprojecting pads of a low friction material that raise the bottom surfacea short distance above the work surface of a cooperating mouse pad.Centrally located within the bottom surface of the mouse is a holethrough which a portion of the underside of a rubber-surfaced steel ball(hereinafter called simply a rubber ball) extends; in operation gravitypulls the ball downward and against the top surface of the mouse pad.The mouse pad is typically a closed cell foam rubber pad covered with asuitable fabric. The low fiction pads slide easily over the fabric, butthe rubber ball does not skid, but instead rolls as the mouse is movedinterior to the mouse are rollers, or wheels, that contact the ball atits equator (the great circle parallel to the bottom surface of themouse) and convert its rotation into electrical signals. The externalhousing of the mouse is shaped such that when it is covered by theuser's hand it appears to have a “front-to-back” axis (along the user'sforearm) and an orthogonal “left-to-right” axis. The interior wheelsthat contact the ball's equator are arranged so that one wheel respondsonly to rolling of the ball that results from a motion component of themouse that is along the front-to-back axis, and also so that the otherwheel responds only to rolling produced by a motion component along theleft-to-right axis. The resulting rotations of the wheels or contactrollers produce electrical signals representing these motion components.(Say, F/B representing Forward and Backward, and L/R representing Leftor Right.) These electrical signals F/B and L/R are coupled to thecomputer, where software responds to the signals to change by a Δx and aΔy the displayed position of a pointer (cursor) in accordance withmovement of the mouse. The user moves the mouse as necessary to get thedisplayed pointer into a desired location or position. Once the pointeron the screen points at an object or location of interest, one of one ormore buttons on the mouse is activated with the fingers of the handholding the mouse. The activation serves as an instruction to take someaction, the nature of which is defined by the software in the computer.

Unfortunately, the usual sort of mouse described above is subject to anumber of shortcomings. Among these are deterioration of the mouse ballor damage to its surface, deterioration or damage to the surface of themouse pad and degradation of the ease of rotation for the contactrollers (say, (a) owing to the accumulation of dirt or of lint, or (b)because of wear, or (c) both (a) and (b)). All of these things cancontribute to erratic or total failure of the mouse to perform asneeded. These episodes can be rather frustrating for the user, whosecomplaint might be that while the cursor on the screen moves in allother directions, he can't get the cursor to, say, move downwards.Accordingly, industry has responded by making the mouse ball removablefor easy replacement and for the cleaning of the recessed region intowhich it fits. Enhanced mouse ball hygiene was also a prime motivationin the introduction of mouse pads. Nevertheless, some users becomeextremely disgusted with their particular mouse of the moment when theseremedies appear to be of no avail. Mouse and mouse pad replacement is alively business.

The underlying reason for all this trouble is that the conventionalmouse is largely mechanical in its construction and operation, andrelies to a significant degree on a fairly delicate compromise about howmechanical forces are developed and transferred.

There have been several earlier attempts to use optical methods asreplacements for mechanical ones. These have included the use of photodetectors to respond to mouse motion over specially marked mouse pads,and to respond to the motion of a specially striped mouse ball. U.S.Pat. No. 4,799,055 describes an optical mouse that does not require anyspecially pre-marked surface. (Its disclosed two orthogonal one pixelwide linear arrays of photo sensors in the X and Y directions and itsstate-machine notion detection mechanism make it a distant early cousinto the technique of the incorporated patents, although it is our viewthat the shifted and correlated array [pixel pattern within an area]technique of the incorporated patents is considerably more sophisticatedand robust.) To date, and despite decades of user frustration with themechanical mouse, none of these earlier optical techniques has beenwidely accepted as a satisfactory replacement for the conventionalmechanical mouse. Thus, it would be desirable if there were anon-mechanical mouse that is viable from a manufacturing perspective,relatively inexpensive, reliable, and that appears to the user asessentially the operational equivalent of the conventional mouse. Thisneed could be met by a new type of optical mouse has a familiar “feel”and is free of unexpected behaviors. It would be even better if theoperation of this new optical mouse did not rely upon cooperation with amouse pad, whether special or otherwise, but was instead able tonavigate upon almost any arbitrary surface.

SUMMARY OF THE INVENTION

In an embodiment, a method of tracking movement relative to a surfaceinvolves providing a source of non-coherent light for illuminating asurface at a low angle of incidence, providing a two dimensional arrayof photo detectors for producing arrays of data elements responsive tolight reflected from surface irregularities on the illuminated surface,providing circuitry for capturing a first array of data elements fromthe two dimensional array of photo detectors at a first time, forexamining the first array of data elements to determine whether or not acharacteristic condition exists in the first array of data elements, forcapturing a second array of data elements from the two dimensional arrayof photo detectors at a second time, and for tracking movement relativeto the surface by comparing at least some of the data elements of thefirst array with at least some of the data elements of the second arrayif it is determined, from the examination of the first array of dataelements, that the characteristic condition does not exist in the firstarray of data elements, wherein the characteristic condition is relatedto the uniformity of at least a majority of the data elements of thefirst array.

In an embodiment, providing the circuitry further involves providingcircuitry for capturing a third array of data elements from the twodimensional array of photo detectors at a third time if thecharacteristic condition is detected in the first array, and fortracking movement relative to the surface by comparing at least some ofthe data elements of the second array with at least some of the dataelements of the third array.

In an embodiment, the angle of incidence is a grazing angle.

In an embodiment, the angle of incidence is between about 5° and about20°.

In an embodiment, providing the circuitry further involves providingcircuitry for capturing a fourth array of data elements from the twodimensional array of photo detectors at a fourth time if thecharacteristic condition is not detected in the data elements of thefirst array, and then tracking movement relative to the surface bycomparing at least some of the data elements of the first array with atleast some of the data elements of the fourth array.

In an embodiment, a method of tracking movement involves providing asource of non-coherent light for illuminating a surface at a low angleof incidence, providing a two dimensional array of photo detectorsresponsive to light reflected from features on the illuminated surface,and providing circuitry for tracking movement relative to the surface bycomparing light reflected from the features at a first time with lightreflected there from at a second time unless it is determined by anexamination of an array of data elements produced in response to thelight reflected at the first time that light detected by at least amajority of the photo detectors at the first time is at least nearlyuniform.

In an embodiment, a method of tracking movement relative to a surfaceinvolves providing a source of non-coherent light for illuminating asurface at a low angle of incidence, providing a two dimensional arrayof photo detectors responsive to light reflected from features locatedon the illuminated surface, and providing circuitry for trackingmovement relative to the surface by comparing light reflected from thefeatures located on the surface at a first time with light reflectedthere from at a second time unless it is determined by an examination ofan array of data elements produced in response to the light reflected atthe first time that light detected by the photo detectors at the firsttime does not represent a focused image of at least some of the featureslocated on the surface.

In an embodiment, providing circuitry further involves providingcircuitry for thereafter tracking movement relative to the surface bycomparing light reflected from the features at the first time with lightreflected there from at a third time if movement was tracked bycomparing light reflected at the first time with light reflected at thesecond time.

In an embodiment, providing circuitry further involves providingcircuitry for thereafter tracking movement relative to the surface bycomparing light reflected from the features at the second time withlight reflected there from at a third time if movement was not trackedby comparing light reflected at the first time with light reflected atthe second time.

In an embodiment, the angle of incidence is between about 5° and about20°.

In an embodiment, a method of tracking movement involves providing asource of non-coherent light for illuminating a surface at a low angleof incidence, providing a two dimensional array of photo detectors forproducing arrays of data elements responsive to light reflected fromsurface irregularities on the illuminated surface, and providingcircuitry for capturing a first array of data elements from the twodimensional array of photo detectors at a first time, for examining thefirst array of data elements to determine whether or not acharacteristic condition exists in the first array of data elements, forcapturing a second array of data elements from the two dimensional arrayof photo detectors at a second time, and for tracking movement relativeto the surface by comparing at least some of the data elements of thefirst array with at least some of the data elements of the second arrayif it is determined, from the examination of the first array of dataelements, that the characteristic condition does not exist in the firstarray of data elements, wherein the characteristic condition is relatedto a representation of a focused image in the data elements of the firstarray.

In an embodiment, providing the circuitry further involves providingcircuitry for capturing a third array of data elements from the twodimensional array of photo detectors at a third time if thecharacteristic condition is detected in the first array, and fortracking movement relative to the surface by comparing at least some ofthe data elements of the second array with at least some of the dataelements of the third array.

In an embodiment, an imaging and navigation arrangement includes asource of non-coherent light for illuminating a surface at a low angleof incidence, a two dimensional array of photo detectors for producingarrays of data elements responsive to light reflected from surfaceirregularities on the illuminated surface, and circuitry for capturing afirst array of data elements from the two dimensional array of photodetectors at a first time, for examining the first array of dataelements to determine whether or not a characteristic condition existsin the first array of data elements, for capturing a second array ofdata elements from the two dimensional array of photo detectors at asecond time, and for tracking movement relative to the surface bycomparing at least some of the data elements of the first array with atleast some of the data elements of the second array if it is determined,from the examination of the first array of data elements, that thecharacteristic condition does not exist in the first array of dataelements, wherein the characteristic condition is related to theuniformity of at least a majority of the data elements of the firstarray.

In an embodiment, an imaging and navigation arrangement includes asource of non-coherent light for illuminating a surface at a low angleof incidence, a two dimensional array of photo detectors responsive tolight reflected from features on the illuminated surface, circuitry fortracking movement relative to the surface by comparing light reflectedfrom the features at a first time with light reflected there from at asecond time unless it is determined by an examination of an array ofdata elements produced in response to the light reflected at the firsttime that light detected by at least a majority of the photo detectorsat the first time is at least nearly uniform.

In an embodiment, an imaging and navigation arrangement includes asource of non-coherent light for illuminating a surface at a low angleof incidence, a two dimensional array of photo detectors responsive tolight reflected from features located on the illuminated surface, andcircuitry for tracking movement relative to the surface by comparinglight reflected from the features located on the surface at a first timewith light reflected there from at a second time unless it is determinedby an examination of an array of data elements produced in response tothe light reflected at the first time that light detected by the photodetectors at the first time does not represent a focused image of atleast some of the features located on the surface.

In an embodiment, an imaging and navigation arrangement includes asource of non-coherent light for illuminating a surface at a low angleof incidence, a two dimensional array of photo detectors for producingarrays of data elements responsive to light reflected from surfaceirregularities on the illuminated surface, and circuitry for capturing afirst array of data elements from the two dimensional array of photodetectors at a first time, for examining the first array of dataelements to determine whether or not a characteristic condition existsin the first array of data elements, for capturing a second array ofdata elements from the two dimensional array of photo detectors at asecond time, and for tracking movement relative to the surface bycomparing at least some of the data elements of the first array with atleast some of the data elements of the second array if it is determined,from the examination of the first array of data elements, that thecharacteristic condition does not exist in the first array of dataelements, wherein the characteristic condition is related to arepresentation of a focused image in the data elements of the firstarray.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified pictographic cut-away side view of a prior artimaging and navigation arrangement;

FIG. 2 is a bottom view of a mouse constructed in accordance with theinvention;

FIG. 3 is a side perspective view of a mouse constructed in accordancewith one aspect of the invention; and

FIG. 4 is a simplified side cut-away view of a proximity sensor in thebase of the mouse of FIGS. 2 and 3 and used to automatically activate ahold feature;

FIG. 5 is a simplified flow chart describing an aspect of internalseeing eye mouse operation related to the operation of the hold featurewhen used in conjunction with a feature called prediction;

FIG. 6 is a simplified portion of a modification of the flow chart ofFIG. 5 and illustrates the velocity detection method of invoking thehold feature; and

FIG. 7 is a perspective view of a plotted correlation surface that hasgood concavity.

DESCRIPTION OF A PREFERRED EMBODIMENT

An optical mouse images as an array of pixels the spatial features ofgenerally any micro textured or micro detailed work surface below themouse. The photo detector responses are digitized and may be stored as aframe into memory. Motion produces successive frames of translatedpatterns of pixel information, which may be compared by autocorrelationto ascertain the direction and amount of movement. A hold featuresuspends the production of movement signals to the computer, allowingthe mouse to be physically relocated on the work surface withoutdisturbing the position on the screen of the pointer. This may be neededif the operator runs out of room to physically move the mouse further,but the screen pointer still needs to go further. The hold feature maybe implemented with an actual button, a separate proximity detector orby detecting the presence of a characteristic condition in the digitizeddata, such as loss of correlation or velocity in excess of a selectedlimit. A convenient place for an actual hold button is along the sidesof the mouse near the bottom, where the thumb and the opposing ringfinger grip the mouse. The gripping force used to lift the mouse engagesthe hold function. Hold may incorporate a brief delay upon either therelease of the hold button, detection of proper proximity or the returnof reasonable digitized values. During that delay any illuminationcontrol or AGC servo loops stabilize. A new reference frame may be takenprior to the resumption of motion detection.

A solution to the problem of replacing a conventional mechanical mousewith an optical counterpart is to optically detect motion by directlyimaging, as an array of pixels, the various particular spatial featuresof a work surface below the mouse, much as human vision is believed todo. In general, this work surface may be almost any flat surface; inparticular, the work surface need not be a mouse pad, special orotherwise. To this end the work surface below the imaging mechanism maybe illuminated from the side, say, with an infrared (IR) light emittingdiode (LED). A surprisingly wide variety of surfaces create a richcollection of highlights and shadows when illuminated with a suitableangle of incidence. That angle is generally low, say, on the order offive to twenty degrees, and we shall term it a “grazing” angle ofincidence. Paper, wood, formica and painted surfaces all work well;about the only surface that does not work is smooth glass (unless it iscovered with fingerprints!). The reason these surfaces work is that theypossess a micro texture, which in some cases may not be perceived by theunaided human senses.

IR light reflected from the micro textured surface is focused onto asuitable array (say, 16×16 or 24×24) of photo detectors. The LED may becontinuously on with either a steady or variable amount of illuminationservoed to maximize some aspect of performance (e.g., the dynamic rangeof the photo detectors in conjunction with the albedo of the worksurface). Alternatively, a charge accumulation mechanism coupled to thephoto detectors may be “shuttered” (by current shunting switches) andthe LED pulsed on and off to control the exposure by servoing theaverage amount of light. Turning the LED off also saves power; animportant consideration in battery operated environments. The responsesof the individual photo detectors are digitized to a suitable resolution(say, six or eight bits) and stored as a frame into correspondinglocations within an array of memory. Having thus given our mouse an“eye”, we are going to further equip it to “see” movement by performingcomparisons with successive frames.

Preferably, the size of the image projected onto the photo detectors maybe a slight magnification of the original features being imaged, say, bytwo to four times. However, if the photo detectors are small enough itmay be possible and desirable to dispense with magnification. The sizeof the photo detectors and their spacing may be such that there is muchmore likely to be one or several adjacent photo detectors per imagefeature, rather than the other way around. Thus, the pixel sizerepresented by the individual photo detectors corresponds to a spatialregion on the work surface of a size that is generally smaller than thesize of a typical spatial feature on that work surface, which might be astrand of fiber in a cloth covering a mouse pad, a fiber in a piece ofpaper or cardboard, a microscopic variation in a painted surface, or anelement of an embossed micro texture on a plastic laminate. The overallsize of the array of photo detectors is preferably large enough toreceive the images of several features. In this way, images of suchspatial features produce translated patterns of pixel information as themouse moves. The number of photo detectors in the array and the framerate at which their contents are digitized and captured cooperate toinfluence how fast the seeing-eve mouse can be moved over the worksurface and still be tracked. Tracking may be accomplished by comparinga newly captured sample frame with a previously captured reference frameto ascertain the direction and amount of movement. One way that may bedone is to shift the entire content of one of the frames by a distanceof one pixel (corresponds to a photo detector), successively in each ofthe eight directions allowed by a one pixel offset trial shift (oneover, one over and one down, one down, one up, one up and one over, oneover in the other direction, etc.). That adds up to eight trials, but wemustn't forget that there might not have been any motion, so a ninthtrial “null shift” may also be required. After each trial shift thoseportions of the frames that overlap each other are subtracted on a pixelby pixel basis, and the resulting differences are (preferably squaredand then) summed to form a measure of similarity (correlation) withinthat region of overlap. Larger trial shifts are possible, of course(e.g., two over and one down), but at some point the attendantcomplexity ruins the advantage, and it is preferable to simply have asufficiently high frame rate with small trial shifts. The trial shiftwith the least difference (greatest correlation) can be taken as anindication of the motion between the two frames. That is, it provides araw F/B and L/R. The raw movement information may be scaled and oraccumulated to provide display pointer movement information (Ax and Av)of a convenient granularity and at a suitable rate of informationexchange.

The actual algorithms described in the incorporated patents (and used bythe mouse) are refined and sophisticated versions of those describedabove. For example, let us say that the photo detectors were a 16×16array. We could say that we initially take a reference frame by storingthe digitized values of the photo detector outputs as they appear atsome time t-o. At some later time t-1 we take a sample frame and storeanother set of digitized values. We wish to correlate a new collectionof nine comparison frames (thought to be, null, one over, one over andone up, etc.) against a version of the reference frame representing“where we were last time”. The comparison frames are temporarily shiftedversions of the sample frame; note that when shifted a comparison framewill no longer overlap the reference frame exactly. One edge, or twoadjacent edges will be unmatched, as it were. Pixel locations along theunmatched edges will not contribute to the corresponding correlation(i.e., for that particular shift), but all the others will. And thoseothers are a substantial number of pixels, which gives rise to a verygood signal to noise ratio. For “nearest neighbor” operation (i.e.,limited to null, one over, one up/down, and the combinations thereof)the correlation produces nine “correlation values”, which may be derivedfrom a summing of squared differences for all pixel locations havingspatial correspondence (i.e., a pixel location in one frame that isindeed paired with a pixel location in the other frame—unmatched edgeswon't have such pairing).

A brief note is perhaps in order about how the shifting may be done andthe correlation values obtained. The shifting may be accomplished byaddressing offsets to memories that can output an entire row or columnof an array at one time. Dedicated arithmetic circuitry may be connectedto the memory array that contains the reference frame being shifted andto the memory array that contains the sample frame. The formulation ofthe correlation value for a particular trial shift (member of thenearest or near neighbor collection) is accomplished very quickly. Thebest mechanical analogy is to imagine a transparent (reference) film ofclear and dark patterns arranged as if it were a checker board, exceptthat the arrangement is perhaps random. Now imagine that a second(sample) film having the same general pattern is overlaid upon thefirst, except that it is the negative image (dark and clear areinterchanged). Now the pair is aligned and held up to the light. As thereference film is moved relative to the sample film the amount of lightadmitted through the combination will vary according to the degree thatthe images coincide. The positioning that admits the least light is thebest correlation. If the negative image pattern of the reference film isa square or two displaced from the image of the sample film, thepositioning that admits the least light will be one that matches thatdisplacement. We take note of which displacement admits the least light;for the mouse we notice the positioning with the best correlation andsay that the mouse moved that much. That, in effect, is what happenswithin an integrated circuit (IC) having photo detectors, memory andarithmetic circuits arranged to implement the image correlation andtracking technique we are describing.

It would be desirable if a given reference frame could be re-used withsuccessive sample frames. At the same time, each new collection of nine(or twenty-five) correlation values (for collections at t₁, t_(i-1),etc.) that originates from a new image at the photo detectors (a nextsample frame) should contain a satisfactory correlation. For a hand heldmouse, several successive collections of comparison frames can usuallybe obtained from the (16×16) reference frame taken at t-o. What allowsthis to be done is maintaining direction and displacement data for themost recent motion (which is equivalent to knowing velocity and timeinterval since the previous measurement). This allows “prediction” ofhow to (permanently!) shift the collection of pixels in the referenceframe so that for the next sample frame a “nearest neighbor” can beexpected to correlate. This shifting to accommodate prediction throwsaway, or removes, some of the reference frame, reducing the size of thereference frame and degrading the statistical quality of thecorrelations. When an edge of the shifted and reduced reference framebegins to approach the center of what was the original reference frameit may be time to take a new reference frame. This manner of operationmay be termed “prediction” and could also be used with comparison framesthat are 5×5 and an extended “near neighbor” (null, two over/one up, oneover/two up, one over/one up, two over, one over, . . . ) algorithm. Thebenefits of prediction are a speeding up of the tracking process bystreamlining internal correlation procedure (avoiding the comparison oftwo arbitrarily related 16×16 arrays of data) and a reduction of thepercentage of time devoted to acquiring reference frames.

In addition to the usual buttons that a mouse generally has, the mousemay have another button that suspends the production of movement signalsto the computer, allowing the mouse to be physically relocated on thework surface without disturbing the position on the screen of thepointer. This may be needed if the operator runs out of room tophysically move the mouse further, but the screen pointer still needs togo further. This may happen, say, in a UNIX system employing a displaysystem known as “Single Logical Screen” (SLS) where perhaps as many asfour monitors are arranged to each display some subportion of theoverall “screen”. If these monitors were arranged as one high by fouracross, then the left to right distance needed for a singlecorresponding maximal mouse movement would be much wider than usuallyallowed for. The usual maneuver executed by the operator for, say, anextended rightward excursion, is to simply pick the mouse up at theright side of the work surface (a mouse pad, or perhaps simply the edgeof clearing on an otherwise cluttered surface of his desk), set it downon the left and continue to move it to the right. What is needed is away to keep the motion indicating signals from undergoing spuriousbehavior during this maneuver, so that the pointer on the screen behavesin an expected and non-obnoxious manner. The function of the “hold”button may be performed automatically by a proximity sensor on theunderside of the mouse that determines that the mouse is not in contactwith the work surface, or by noticing that all or a majority of thepixels in the image have “gone dark” (it's actually somewhat morecomplicated than that—we shall say more about this idea in the nextparagraph). Without a hold feature, there may be some slight skewing ofthe image during the removal and replacement of the mouse, owing either:(a) to a tilting of the field of view as the mouse is lifted; or (b) tosome perverse mistake where frames for two disparate and widelyseparated spatial features imaged at very different times during theremoval and replacement are nevertheless taken as representing a smalldistance between two frames for the same feature. A convenient place foran actual hold button is along the sides of the mouse near the bottom,where the thumb and the opposing ring finger would grip the mouse tolift it up. A natural increase in the gripping force used to lift themouse would also engage the hold function. A hold feature mayincorporate an optional brief delay upon either the release of the holdbutton, detection of proper proximity or the return of reasonabledigitized values. During that delay any illumination control servo loopsor internal automatic gain controls would have time to stabilize and anew reference frame would be taken prior to the resumption of motiondetection.

And now for this business of the pixels in the image “going dark”. Whathappens, of course, may be that the IR light from the illuminating LEDno longer reaches the photo detectors in the same quantity that it did,if at all; the reflecting surface may be too far away or may be simplynot in view. However, if the mouse were turned over, or its undersideexposed to an intensely lit environment as a result of its being lifted,then the outputs of the photo detectors might be at any level. The keyis that they will be uniform, or nearly so. The main reason that theybecome uniform is that there is no longer a focused image; all the imagefeatures are indistinct and they are each spread out over the entirecollection of photo detectors. So the photo detectors uniformly come tosome average level. This is in distinct contrast with the case whenthere is a focused image. In the focused case the correlations betweenframes (recall the one over, one over and one down, etc.) exhibit adistinct phenomenon.

Assume that the spatial features being tracked mapped exactly onto thephoto detectors, through the lens system, and that mouse movement werejerky by exactly the amount and in the directions needed for a featureto go from detector to detector. Now for simplicity assume also thatthere is only one feature, and that its image is the size of a photodetector. So, all the photo detectors but one are all at pretty much thesame level, and the one detector that is not at that level is at asubstantially different level, owing to the feature. Under these highlyidealized conditions it is clear that the correlations will be very wellbehaved; eight “large” differences and one small difference (a sink holein an otherwise fairly flat surface) in a system using nine trials for anearest neighbor algorithm (and remembering that there may have been nomotion). Note: The astute reader will notice that the “large” differencein this rather contrived example actually corresponds to, or originateswith, only one pixel, and probably does not deserve to be called“large”—recall the earlier shifted film analogy. The only light passedby the films for this example would be for the one pixel of the feature.A more normal image having a considerably more diverse collection ofpixels increases the difference to where it truly is a “large”difference.

Now, such highly idealized conditions are not the usual case. It is morenormal for the image of the tracked spatial features to be both largerand smaller than the size of the photo detectors, and for the mousemotion to be continuous, following a path that allows those images tofall onto more than one detector at once. Some of the detectors willreceive only a partial image, which is to say, some detectors willperform an analog addition of both light and dark. The result may be atleast a “broadening” of the sink hole (in terms of the number of photodetectors associated with it) and very possibly a corresponding decreasein the depth of the sink hole. The situation may be suggested byimagining a heavy ball rolling along a taut but very stretchablemembrane. The membrane has a discrete integer Cartesian coordinatesystem associated with it. How much does the membrane distend at anyinteger coordinate location as the ball rolls? First imagine that theball is of a very small diameter but very heavy, and then imagine thatthe ball is of a large diameter, but still weighs the same. The analogymay not be exact, but it serves to illustrate the idea of the “sinkhole” mentioned above. The general case is that the generally flatsurface with sharply defined sink hole becomes a broad concavity, orbowl.

We shall term the surface produced or described by the variouscorrelation values the “correlation surface” and will, at various times,be most interested in the shape of that surface.

We say all of this to make two points. First, the shifting shape of theconcavity in the correlation surface as the mouse moves allowsinterpolation to a granularity finer than the simple size/spacing of thephoto detectors. We point this out, with the remark that our mouse cando that, and leave it at that. The full details of interpolation aredescribed in the incorporated patents. No further discussion ofinterpolation is believed necessary. Second, and this is our real reasonfor the discussion of the preceding paragraphs, is the observation thatwhat happens when the mouse is picked up is that the concavity in thecorrelation surface goes away, to be replaced by generally equal valuesfor the correlations (i.e., a “flat” correlation surface). It is whenthis happens that we may say with considerable assurance that the mouseis air borne, and can then automatically invoke the hold feature, untilafter such time that a suitable concavity (“bowl”) reappears.

Another method for invoking or initiating a hold feature is to simplynotice that the mouse is moving faster than a certain threshold velocity(and is thus presumably experiencing an abrupt retrace motion in amaneuver intended to translate the screen pointer further than theavailable physical space within which the mouse is operating). Once thevelocity threshold is exceeded the motion indicating signals that wouldotherwise be associated with that movement are suppressed until suchtime as the velocity drops below a suitable level.

Refer now to FIG. 1, wherein is shown a simplified representation of acut-away side view of a prior art imaging and navigation arrangement 1that is generally of the type described by the incorporated patents. AnLED 2, which may be an IR LED, emits light which is projected by lens 3(which instead of being separate may be an integral part of the LED'Spackage), through orifice 13 in bottom surface 6 and onto a region 4that is part of a work surface 5. The average angle of incidence ispreferably within the range of five to twenty degrees. Although it hasbeen omitted for clarity, the orifice 13 might include a window that istransparent for the light from LED 2, and which would serve to keepdust, dirt or other contamination out of the innards of the seeing eyemouse. Work surface 5 might belong to a special object, such as a mousepad, or more generally, it will not, and might be the surface of nearlyanything except smooth glass. Examples of suitable materials include,but are not limited to, paper, cloth, laminated plastic tops, paintedsurfaces, frosted glass (smooth side down, thank you), desk pads, realwood, fake wood, etc. Generally, any micro textured surface havingfeatures whose size falls within the range of 5 to 500 microns will do.

The illumination of micro textured surfaces is most effective when donefrom the side, as this accentuates the pattern of highlights and shadowsproduced by surface height irregularities. Suitable angles of incidencefor illumination cover the range of about five to twenty degrees. A verysmooth or flat surface (e.g., one that has been ground and polished)having simple variations in reflectivity owing to (micro scale)compositional variation works, too. In such a case (and assuming that itcan be guaranteed) the angle of incidence for the illumination couldapproach ninety degrees, since the urge to create shadows goes away.However, such a smooth yet micro detailed surface is not what we wouldordinarily think of when we say “arbitrary surface”, and a seeing eyemouse intended for use on an “arbitrary surface” that is more likelymicro textured would work best if equipped to provide a grazing angle ofincident illumination.

An image of the illuminated region 4 is projected through an opticalwindow 9 in package portion 8 a of an integrated circuit and onto anarray 10 of photo detectors. This is done with the aid of lens 7. Thepackage portion 8 a might also dispense with separate window 9 and lens7 by combining them into one and the same element. The photo detectorsmay comprise a square array of, say, 12 to 24 detectors on a side, eachdetector being a photo transistor whose photo sensitive region is 45 by45 microns and of 60 microns center to center spacing. The phototransistors charge capacitors whose voltages are subsequently digitizedand stored in a memory. The array 10 is fabricated onto a portion of anintegrated circuit die 12 affixed by an adhesive 11 onto package portion8 b. What is not shown are any of the details of how the integratedcircuit is held in place (probably by a printed circuit board), theshape or composition of the lenses, or of how the lenses are mounted; itis clear that those things are doable in a conventional manner. It isalso clear that the general level of illumination of region 4 may becontrolled by noticing the output levels of the photo detectors andadjusting the intensity of light issuing from the LED 2. This could beeither continuous control or pulse width modulation, or some combinationof both.

Once again, the reader is reminded that the details of the motionsensing operation are thoroughly described in the incorporated patents(and briefly described in the Summary); accordingly, they need not berepeated here.

Refer now to FIG. 2, which is a bottom view of a mouse 14 constructed inaccordance with the invention. In short, this bottom view of thisparticular seeing eye mouse 14 looks very similar to the bottom view ofa particular conventional mouse from Hewlett-Packard Co., to wit: theC1413A. The major difference is that where there ought to be a ballthere is a protective lens or window 16 that is transparent to IR light.This is the omitted transparent window in orifice 13 that was mentionedin the description of FIG. 1. Also missing is the usual rotatableannulus that serves as a removable retainer 10 allow access to the ballfor cleaning or replacement. What is shown in the figure is theunderside IS of the mouse 14 (corresponds to 6 in FIG. 1), low frictionglides 19 and connecting cable 17 with its strain relief 18. Of course,our seeing eye mouse 14 could be a cordless mouse, as well, with anoptical or radio communication link to the computer.

Refer now to FIG. 3, wherein is shown a side perspective view of a mouse14 constructed in accordance with one aspect of the invention. Thataspect of the invention is the hold feature. The hold feature is anaspect of seeing eye mouse operation that suspends the production ofmovement information or signals to the computer when it is determinedthat the mouse is not suitably proximate to the work surface whosespatial features are being tracked. This allows the seeing eye mouse tobe picked up, translated and set back down, or, as we shall term such anoperation, “swiped” across the work surface.

In particular, the seeing eye mouse 14 in FIG. 3 includes at least onehold button 24 located in side skirt 20 near the bottom surface 15 so asto be beneath the right thumb or the left ring finger, depending uponwhich hand the operator is using. There may be another symmetricallylocated button on the other side (not shown) that would contact eitherthe left thumb or the right ring finger.

The mouse 14 conventionally includes a surface 21 which nestles in thepalm of the hand, and first and second “regular” mouse buttons 22 and 23that are actuated by the index and middle fingers. These operate intheir normal fashion.

Button or buttons 24 are activated by a natural increase in the grippingforce needed to pick the mouse 14 up during a swipe. When one or both ofthese button are pressed the hold feature is activated. For the durationof the hold the sending of motion signals to the computer is suspended.When the hold is over (the buttons are released) a new reference frameis taken before any new motion signals are sent to the computer. Thisallows swiping, and has the advantage that the user has the ability toexpressly force the onset of the hold feature.

The hold feature could also be automatically activated by the action ofa separate proximity sensor n the bottom of the mouse. This is what isshown in FIG. 4, where a shouldered aperture 26 in the base 6 receives ashouldered plunger 25 made captive by the lever arm of a switch 28above. The switch 8 is activated by movement of the plunger 25, suchthat when the plunger moves significantly in the direction of arrow 27the hold feature is activated. The exact nature of the separateproximity sensor is matter of choice, and while it could be a simple asthe micro switch 28 operated by the weight of the mouse through theplunger 25, other, non-mechanical, methods are also possible.

Yet another way to automatically activate and deactivate the holdfeature is to examine the nature of the digitized data of the array 10of photo detectors. When the outputs of the photo detectors becomesufficiently uniform it may be surmised that there is no longer an imagewith variations projected onto the array 10 of photo detectors. Thisuniformity will reveal itself by producing a correlation surface that isflat, or nearly so. Rather than separately detecting uniform levels(which would use hardware not otherwise present), we prefer instead toexamine the shape of the correlation surface, (which surface we need forother reasons, anyway). The most probable cause of a flat correlationsurface is that the mouse has been picked up. This mode of operation mayrequire that there be a fairly narrow depth of field, lest here occurundue delay in activating the hold. Such delay could produce minorartifacts in screen pointer movement. These might include slightunintended screen pointer movements owing to tilting of the mouse as itis either picked up or replaced. As long as activating the hold feature(however done, whether manually or automatically) forces acquisition ofa new reference frame before resuming the production of motion signals,there should be no danger of producing a spurious indication resultingfrom the combination of old data with some new data that justaccidentally looks like a proper small motion in some inappropriatedirection. However, with mere level detection (of, say, a sample frame)it may be difficult to guarantee that while in motion in the air thereoccur no optical effects (a reflection of a bright source) that wouldconfuse the algorithm. It will be appreciated that the shape of thecorrelation surface is a much more reliable indicator. All of that said,it must still be remembered that the steering, as it were, of the screenpointer is an incrementally driven servo-like operation performed by ahuman being; if the screen pointer isn't there yet, just keep moving themouse as needed! Small perturbations during swiping are not fatal, andmay not even be particularly noticeable, depending upon the specificapplication being performed.

Refer now to FIG. 5, wherein is shown a flow chart 29 that describes anaspect of seeing eye mouse operation involving the hold and predictionproperties. We may assume that there is some start condition or location30, from which is reached step 31: ACQUIRE A REFERENCE FRAME. Thisrefers to illuminating the LED 2 and storing a collection of digitizedphoto detector values into an array of memory (not shown). The next step32 is ACQUIRE A SAMPLE FRAME. This refers to the same actions, exceptthat the data is stored in a different array of memory, and may reflectmouse motion relative to where it was when step 31 was performed. Atstep 33, COMPUTE CORRELATION VALUES, the nine (or perhaps twenty-five)correlation values are quickly computed by some heavy duty dedicatedarithmetic hardware assisted by automatic address translation and a verywide path out of the memory arrays. At step 34, IS THE CORRELATIONSURFACE SUITABLY CONCAVE?, the nature of the correlation surfacedescribed by the collection of correlation values computed in step 33 isexamined. We want to know if it is shaped like a bowl, and if so, “howmuch water will it hold,” so to speak

If the shape of the correlation surface is a good bowl, then path 36takes us to the optional step 37: IS THE HOLD BUTTON PRESSED?; moreabout that in the next paragraph. Otherwise, we have a flat correlationsurface, or a “bad bowl,” and proceed along path 35 to optional step 42,DELAY. There are several possible causes for this exit from qualifier34: e.g., extreme velocity, a suddenly featureless work surface, and, anairborne mouse. In the absence of an explicit HOLD button, we will relyupon exit path 35 to provide proper seeing eye mouse behavior bysuppressing motion signs to the computer during the airborne portion ofa swiping operation.

If the seeing eye mouse does have a HOLD button, then optional qualifier37 is present, and it is there that the status (pressed or not) of theHOLD 24 button is determined. The case where it is pressed is treatedthe same as that for a bad bowl at qualifier 34. That is, path 38 istaken, which also leads to optional step 42.

Optional step 42 provides a delay which may be useful in several ways.First, if there is a swipe in progress, then it takes some time, and bynot imagining during that time some battery power can be saved. Also,suppose that the nature of the delay is slightly more complex than apause in the motion of a moving finger on the flow chart. Suppose thatthe ACQUIRE REFERENCE FRAME step 31 were influenced by there having beena delay at step 42, in that part way through the delay an illuminationlevel control operation is initiated. This could allow time forre-adjustment of illumination levels, and so forth. Whether or not thereis a DELAY at optional step 42, path 43 leads back to step 31, whereanother notion detection cycle begins.

To resume, path 39 leads to step 40: PREDICT SHIFT IN REFERENCE FRAME.As mentioned above, it is generally not necessary to obtain and maintainactual velocities in X and Y, and time interval information, to find thedisplacement needed for prediction. One can imagine measurementenvironments where that might be. needed, but the one shown here is notone of them. Instead, the predicted shift can be taken as the amount ofmovement corresponding to the correlation a! the preceding step 34.

The next step 44 is OUTPUT ΔX & ΔY. It is here that we take note of howmuch mouse motion here has been since the last measurement cycle. Theamount of shift needed to attain correlation is the desired amount.These values may be found by noticing which comparison frame actuallycorrelated assuming no interpolation). These “raw” ΔX and ΔY motionvalues may be accumulated into running values that are sent to thecomputer at a lower rate than that at which the raw values of step 44are produced.

At qualifier 45 we ask if we NEED A NEW REFERENCE FRAME? If the answeris YES, then path 46 leads to step 48: STORE PRESENT SAMPLE FRAME TNREFERENCE FRAME. (A little thought will confirm that this re-use of thesample frame cooperates with not having to maintain actual velocitiesand time intervals for the prediction process. If we took a separate newreference frame it would complicate a lot of things, and would probablyforce the use of D=RT—i.e., the distance formula—for prediction.)

We need a new reference frame when there has been enough shifting of it,owing to predictions, that not enough of it overlaps the comparisonframes for reliable correlations. Somewhere in the range of three tofive shifts (that do not retrace themselves) is about the limit for a16×16 reference frame.

If the answer to qualifier 45 is NO, and we do not need to replace thereference frame, then path 47 takes us to the same step 49 as does thepath leading from step 48. Step 49, SHIFT REFERENCE FRAME, performs theactual permanent shift of the values in the memory array representingthe reference frame. The shift is by the prediction amount, and datashifted away is lost. Following the shifting of the reference frame path50 returns to step 32, ACQUIRE A SAMPLE FRAME, where the nextmeasurement cycle begins.

Refer now to FIG. 6, wherein is shown a simplified flow chart segment 50that shows how to replace step 44 of the flow chart 29 in FIG. 5 withsteps 51-55. The purpose for doing this is similar to the variousmanners of hold operation already described, and may be used inconjunction therewith, or instead thereof. The general idea of themodification represented by FIG. 6 is to outfox the computer by eithernot sending any updated information by skipping step 55A or (optionally,with step 55B) sending zeros for ΔX and ΔY, even when that is not true.This is done whenever step 52 ascertains that the rate of mouse motionexceeds, say, three to six inches per second. For a given seeing eyemouse such a limit is easily expressed as a displacement by a certainnumber of pixels within some number of measurement cycles, assuming thatthe measurement cycle rate is fast compared to normal mouse motion. Theidea is that normal casual mouse motion probably will not require eithera new nearest neighbor reference frame (let alone a maximally shiftedone for 5×5 near neighbor operation) every measurement cycle for somelarge (say, ten to twenty-five) number of consecutive measurementcycles. For if that were the case, the seeing eye mouse would beoperating on the hairy edge of the hold mode via a NO answer toqualifier 34 and path 35. (According to the assumption, any highervelocity will result in loss of correlation!) That is, the expectationis that taking a new reference frame is normally much less frequent. Ofcourse, it could happen that the velocity of the mouse is really high,and path 35 gets used, anyway. That is as it should be. But if themeasurement cycle rate is not sufficiently high with respect to normalexpected mouse motion, then it might not be appropriate to use thetechnique of FIG. 6.

Step 51 represents anything in the old step 44 over and above the actualcommunication to the computer of the values ΔX and ΔY. A tricky exampleof this difference might be an internal accumulation of motion that hasnot yet be dispatched to the computer, owing to a higher internal motionmeasurement cycle rate for the seeing eye mouse than the informationexchange rate with the computer. Now, it may well be the case that insome systems this accumulated information is used for internal mousepurposes other than strictly for keeping the computer informed. If so,then it would need to be preserved, for all that qualifier 52, path 53(and bypassed step 55A) need to accomplish is NOT tell the computerthere has been motion; we want to fool the computer but without makingthe mouse lose its mind.

It will be noticed that if such an accumulation were allowed tocontinued during a rapid retrace, intended to mimic picking the mouseup, the computer might still win in the end when the velocity drops tonormal amounts and the accumulation is finally sent; the screen cursorcould snap to the correct location, anyway, depending upon how theoverall system works. In such a case a separate set of accumulationsshould be maintained, with those for the computer remaining in bypassedstep 55A.

Of course, it may be the case that there is no internal use by the mouseof accumulated ΔX and ΔY, other than to send it to the computer. In thatcase nothing needs to be done, other than to leave that accumulation inbypassed step 55A. It is also possible that in the mouse there simplyarc no accumulations to cause such concerns; say, any such accumulationswere done by software in the computer.

Finally, refer now to FIG. 7. It is a plot 56 of a near neighbor (5×5)correlation surface 57 having a suitable concavity. The two horizontalaxes 58 and 59 represent the X and Y axes of mouse motion; the unitsindicated along the axes are pixels. Drawn onto the plane of the axes 58and 59 are smoothed and interpolated contour lines 60 intended tofurther indicate the shape of the correlation surface directly above.The vertical axis 61 a measure of correlation expressed in essentiallyarbitrary units.

1. A method of tracking movement relative of an optical mouse to asurface, comprising: providing a source of non-coherent light forilluminating a surface at a low angle of incidence; providing a twodimensional array of photo detectors for producing arrays of dataelements responsive to light reflected from surface irregularities onthe illuminated surface; and providing circuitry for capturing a firstarray of data elements from the two dimensional array of photo detectorsat a first time, for examining the first array of data elements todetermine whether or not a characteristic condition exists in the firstarray of data elements; for capturing a second array of data elementsfrom the two dimensional array of photo detectors at a second time, andfor tracking movement relative to the surface by comparing at least someof the data elements of the first array with at least some of the dataelements of the second array if it is determined, from the examinationof the first array of data elements, that the characteristic conditiondoes not exist in the first array of data elements, wherein thecharacteristic condition is related to the uniformity of at least amajority of the data elements of the first array.
 2. The method of claim1, wherein providing the circuitry further comprises: providingcircuitry for capturing a third array of data elements from the twodimensional array of photo detectors at a third time if thecharacteristic condition is detected in the first array, and fortracking movement relative to the surface by comparing at least some ofthe data elements of the second array with at least some of the dataelements of the third array.
 3. The method of claim 1 or 2 wherein theangle of incidence is a grazing angle.
 4. The method of claim 1 or 2wherein the angle of incidence is between about 5 degrees and about 20degrees.
 5. The method of claim 1 or 2 wherein providing the circuitryfurther comprises: providing circuitry for capturing a fourth array ofdata elements from the two dimensional array of photo detectors at afourth time if the characteristic condition is not detected in the dataelements of the first array, and then tracking movement relative to thesurface by comparing at least some of the data elements of the firstarray with at least some of the data elements of the fourth array.
 6. Amethod of tracking movement of an optical mouse, comprising: providing asource of non-coherent light for illuminating a surface at a low angleof incidence; providing a two dimensional array of photo detectorsresponsive to light reflected from irregular surfaces on the illuminatedsurface; and providing circuitry for tracking movement relative to thesurface by comparing light reflected from the irregular surfaces at afirst time with light reflected there from at a second time unless it isdetermined by an examination of an array of data elements produced inresponse to the light reflected at the first time that light detected byat least a majority of the photo detectors at the first time is at leastnearly uniform.
 7. A method of tracking movement of an optical mouserelative to a surface, comprising: providing a source of non-coherentlight for illuminating a surface at a low angle of incidence; providinga two dimensional array of photo detectors responsive to light reflectedfrom irregular surfaces located on the illuminated surface; andproviding circuitry for tracking movement relative to the surface bycomparing light reflected from the irregular surfaces located on thesurface at a first time with light reflected there from at a second timeunless it is determined by an examination of an array of data elementsproduced in response to the light reflected at the first time that lightdetected by the photo detectors at the first time does not represent afocused image of at least some of the irregular surfaces located on thesurface.
 8. The invention method of claim 6 or 7 wherein providingcircuitry further comprises: providing circuitry for thereafter trackingmovement relative to the surface by comparing light reflected from theirregular surfaces at the first time with light reflected there from ata third time if movement was tracked by comparing light reflected at thefirst time with light reflected at the second time.
 9. The method ofclaims 6 or 7 wherein providing circuitry further comprises: providingcircuitry for thereafter tracking movement relative to the surface bycomparing light reflected from the irregular surfaces at the second timewith light reflected there from at a third time if movement was nottracked by comparing light reflected at the first time with lightreflected at the second time.
 10. The method of claims 6 or 7 whereinthe angle of incidence is between about 5 degrees and about 20 degrees.11. A method of tracking movement of an optical mouse, comprising:providing a source of non-coherent light for illuminating a surface at alow angle of incidence; providing a two dimensional array of photodetectors for producing arrays of data elements responsive to lightreflected from surface irregularities on the illuminated surface; andproviding circuitry for capturing a first array of data elements fromthe two dimensional array of photo detectors at a first time, forexamining the first array of data elements to determine whether or not acharacteristic condition exists in the first array of data elements; forcapturing a second array of data elements from the two dimensional arrayof photo detectors at a second time, and for tracking movement relativeto the surface by comparing at least some of the data elements of thefirst array with at least some of the data elements of the second arrayif it is determined, from the examination of the first array of dataelements, that the characteristic condition does not exist in the firstarray of data elements, wherein the characteristic condition is relatedto a representation of a focused image in the data elements of the firstarray.
 12. The method of claim 11, wherein providing the circuitryfurther comprises: providing circuitry for capturing a third array ofdata elements from the two dimensional array of photo detectors at athird time if the characteristic condition is detected in the firstarray, and for tracking movement relative to the surface by comparing atleast some of the data elements of the second array with at least someof the data elements of the third array.
 13. An imaging and navigationarrangement for an optical mouse, comprising: a source of non-coherentlight for illuminating a surface at a low angle of incidence; a twodimensional array of photo detectors for producing arrays of dataelements responsive to light reflected from surface irregularities onthe illuminated surface; and circuitry for capturing a first array ofdata elements from the two dimensional array of photo detectors at afirst time, for examining the first array of data elements to determinewhether or not a characteristic condition exists in the first array ofdata elements; for capturing a second array of data elements from thetwo dimensional array of photo detectors at a second time, and fortracking movement relative to the surface by comparing at least some ofthe data elements of the first array with at least some of the dataelements of the second array if it is determined, from the examinationof the first array of data elements, that the characteristic conditiondoes not exist in the first array of data elements, wherein thecharacteristic condition is related to the uniformity of at least amajority of the data elements of the first array.
 14. An imaging andnavigation arrangement for an optical mouse, comprising: a source ofnon-coherent light for illuminating a surface at a low angle ofincidence; a two dimensional array of photo detectors responsive tolight reflected from irregular surfaces on the illuminated surface; andcircuitry for tracking movement relative to the surface by comparinglight reflected from the irregular surfaces at a first time with lightreflected there from at a second time unless it is determined by anexamination of an array of data elements produced in response to thelight reflected at the first time that light detected by at least amajority of the photo detectors at the first time is at least nearlyuniform.
 15. An imaging and navigation arrangement for an optical mouse,comprising: a source of non-coherent light for illuminating a surface ata low angle of incidence; a two dimensional array of photo detectorsresponsive to light reflected from irregular surfaces located on theilluminated surface; and circuitry for tracking movement relative to thesurface by comparing light reflected from the irregular surfaces locatedon the surface at a first time with light reflected there from at asecond time unless it is determined by an examination of an array ofdata elements produced in response to the light reflected at the firsttime that light detected by the photo detectors at the first time doesnot represent a focused image of at least some of the features locatedon the surface.
 16. An imaging and navigation arrangement for an opticalmouse, comprising: a source of non-coherent light for illuminating asurface at a low angle of incidence; a two dimensional array of photodetectors for producing arrays of data elements responsive to lightreflected from surface irregularities on the illuminated surface; andcircuitry for capturing a first array of data elements from the twodimensional array of photo detectors at a first time, for examining thefirst array of data elements to determine whether or not acharacteristic condition exists in the first array of data elements; forcapturing a second array of data elements from the two dimensional arrayof photo detectors at a second time, and for tracking movement relativeto the surface by comparing at least some of the data elements of thefirst array with at least some of the data elements of the second arrayif it is determined, from the examination of the first array of dataelements, that the characteristic condition does not exist in the firstarray of data elements, wherein the characteristic condition is relatedto a representation of a focused image in the data elements of the firstarray.